Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink, whereas other areas will not accept ink.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on a ink-repelling background.
In the production of common lithographic plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon imagewise exposure of such light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
On the other hand, methods are known for making printing plates involving the use of imaging elements that are heat-sensitive rather than photosensitive. A particular disadvantage of photosensitive imaging elements such as described above for making a printing plate is that they have to be shielded from the light. Furthermore they have a problem of stability of sensitivity in view of the storage stability and they show a lower resolution. The trend towards heat-sensitive printing plate precursors is clearly seen on the market.
For example, Research Disclosure no. 33303 of January 1992 discloses a heat-sensitive imaging element comprising on a support a cross-linked hydrophilic layer containing thermoplastic polymer particles and an infrared absorbing pigment such as e.g. carbon black. By image-wise exposure to an infrared laser, the thermoplastic polymer particles are image-wise coagulated thereby rendering the surface of the imaging element at these areas ink acceptant without any further development. A disadvantage of this method is that the printing plate obtained is easily damaged since the non-printing areas may become ink-accepting when some pressure is applied thereto. Moreover, under critical conditions, the lithographic performance of such a printing plate may be poor and accordingly such printing plate has little lithographic printing latitude.
EP-A-514145 discloses a heat-sensitive imaging element including a coating comprising core-shell particles having a water insoluble heat softenable core component and a shell component which is soluble or swellable in aqueous alkaline medium. Red or infrared laser light directed image-wise at said imaging element causes selected particles to coalesce, at least partially, to form an image and the non-coalesced particles are then selectively removed by means of an aqueous alkaline developer. Afterwards a baking step is performed. However the printing endurance of a so obtained printing plate is low.
EP-A-599510 discloses a heat-sensitive imaging element which comprises a substrate coated with (i) a layer which comprises (1) a disperse phase comprising a water-insoluble heat softenable component A and (2) a binder or continuous phase consisting of a component B which is soluble or swellable in aqueous, preferably aqueous alkaline medium, at least one of components A and B including a reactive group or precursor therefor, such that insolubilisation of the layer occurs at elevated temperature and/or on exposure to actinic radiation, and (ii) a substance capable of strongly absorbing radiation and transferring the energy thus obtained as heat to the disperse phase so that at least partial coalescence of the coating occurs. After image-wise irradiation of the imaging element and developing the image-wise irradiated plate, said plate is heated and/or subjected to actinic irradiation to effect insolubilisation. However the printing endurance of a so obtained printing plate is low.
Furthermore EP-A 952022871.0, 952022872.8, 952022873.6 and 952022874.4 disclose a method for making a lithographic printing plate comprising the steps of (1) image-wise exposing to light a heat-sensitive imaging element comprising (i) on a hydrophilic surface of a lithographic base an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light to heat, said compound being comprised in said image forming layer or a layer adjacent thereto; (2) and developing a thus obtained image-wise exposed element by rinsing it with plain water. During the exposure of such an imaging element the imaging element shows partially ablation resulting in a deterioration of the lithographic properties of a so obtained lithographic plate e.g. a decreased ink acceptance on said ablated areas.
Driographic printing plates comprise highly ink-repellant areas and ink-accepting areas which are commonly formed by a silicon layer. These printing plates operate without the use of a dampening liquid. Driographic printing plates can be prepared using a photographic material that is made image-wise receptive or repellant to ink upon photo-exposure of the photographic material. Also heat-sensitive recording materials are known for preparing driographic printing plates. The surface of these heat-sensitive printing plates can be made image-wise receptive or repellant to ink upon image-wise exposure to heat and/or subsequent development.
For example in DE-A-2512038 there is disclosed a heat mode recording material that comprises on a support carrying or having an ink-accepting surface (i) a heat mode recording layer containing a self oxidizing binder e.g. nitrocellulose and a substance that is capable of converting radiation into heat e.g. carbon black and (ii) a non-hardened silicon layer as a surface layer. The disclosed heat mode recording material is image-wise exposed using a laser and is subsequently developed using a developing liquid that is capable of dissolving the silicon layer in the exposed areas. Subsequent to this development the silicon surface layer is cured. Due to the use of naphta as a developing liquid the process is ecologically disadvantageous. Further since the surface layer is not hardened the heat mode recording material may be easily damaged during handling.
FR-A-1.473.751 discloses a heat mode recording material comprising a substrate having an ink-accepting surface, a layer containing nitrocellulose and carbon black and a silicon layer. After image-wise exposure using a laser the imaged areas are said to be rendered ink-accepting. The decomposed silicon layer is not removed. Ink-acceptance of the obtained plates is poor and the printing properties such as printing endurance and resolution of the copies is rather poor.
Research Disclosure 19201 of April 1980 discloses a heat mode recording material comprising a polyester film support provided with a bismuth layer as a heat mode recording layer and a silicon layer on top thereof. The disclosed heat mode recording material is imaged using an argon laser and developed using hexane.
Furthermore EP-A-573091 discloses a heat mode recording material comprising a substrate having an ink-accepting surface, a recording layer containing a light-to-heat converting compound and a silicone layer. After image-wise exposure using a laser beam the exposed areas are rubbed to remove said ink-repellant surface layer and recording layer.
EP-A-580393 (U.S. Pat. No. 5,339,737) discloses a heat-sensitive material comprising a first and second layer, said first layer is a silicone layer containing an IR-absorbing compound and the first and second layer exhibit different affinities towards a printing liquid (ink and/or adhesive liquid for ink). The lithographic printing plate is imaged by a laser and after exposure the ablated parts are removed in a post-imaging cleaning step.
In the latter discussed systems contamination of the exposure unit and of the printing plate can occur with debris from the laser ablated areas. Also development on the printing press is not likely with this type of printing plates.
The above discussed heat-sensitive systems are mostly developed with ecologically harmful solvents and/or are not suitable for driography and/or have poor printing properties. Thus there is still a need for a heat-sensitive recording material that can easily be processed and that yields printing plates with good or excellent printing properties.